Blank verse is unrhymed iambic pentameter.
Iambic Pentameter
pentameter: five feet
foot: one stressed syllable plus one or more unstressed syllables in a repeating pattern.
iambic: an unstressed syllable followed by a stressed syllable: 
Iambic Words
belief arise defend prepare conceive
Regular Blank Verse Lines
We mourn in black, why mourn we not in blood? (1 Henry VI 1.1.17)
My child is yet a stranger in the world. (Romeo and Juliet 1.2.8)
But soft, methinks I scent the morning air. (Hamlet 1.5.58)
Two Common Variations in Blank Verse
Feminine Ending: The line ends with an extra, unstressed syllable.
Or say ’tis not your seal, not your invention (Twelfth Night 5.1.327)
In night and on the court and guard of safety (Othello 2.3.205)
Initial Stress: The line begins with a single, stressed syllable followed by an anapestic foot (two unstressed syllables followed by one stressed syllable: 
).
Now is the winter of our discontent (Richard III 1.1.1)
Veiling an Indian beauty; in a word, (The Merchant of Venice 3.2.99)
